Recubrimientos poliméricos generadores de oxígeno singlete

SERGIO D. EZQUERRA RIEGA; EMILIANO S. DAL MOLIN; HERNÁN B. RODRÍGUEZ; ENRIQUE SAN ROMÁN

San Miguel de Tucumán

Encuentro; Tercera Reunión de Fotobiólogos Moleculares Argentino (III GRAFOB); 2016

Grupo Argentino de Fotobiología

Coatings based on dye-loaded polymer films capable of yielding singlet molecular oxygen (1O2) upon irradiation can be used for the photodynamic inactivation of microorganisms. The establishment of the conditions that maximize the efficiency of generation of 1O2 at the film surface is relevant for biomedical applications. Most important is the adjustment of the dye concentration to allow a good compromise between substantial light absorption and singlet oxygen quantum yield.Thin films of poly(2-hydroxyethyl methacrylate) (pHEMA) containing Rose Bengal (RB) and of polyvinyl alcohol (PVA) and polyvinyl acetate (PVAc) with Phloxine B (PhB) were spin coated on glass substrates from solutions of their components in appropriate solvents. Dye concentrations spanning at least three orders of magnitude were explored. Films were characterized by absorption and fluorescence spectroscopy and laser flash-photolysis, while singlet oxygen leaving the surface was quantified using a chemical monitor (DPBF). The effect of humidity on fluorescence and singlet oxygen quantum yields was also evaluated. Results are compared with those previously obtained on PhB-pHEMA films.1Changes in absorption and fluorescence spectra of RBpHEMA films at high dye concentrations point to weak dye to dye interactions in the ground and excited states. Fluorescence quantum yields are constant at low concentrations and fall down by self quenching after 0.01 M. Calculated 1O2 quantum yields follow the same trend starting from (0.6 +/-0.2). Results, interpreted in the context of a quenching radius model,1 point to the occurrence of bright traps capable of yielding triplet states and 1O2. The dye concentration which maximizes the 1O2 generation efficiency is about 0.1 M. This value is higher than that obtained for PhB-pHEMA films (about 0.02 M), for which traps are non-fluorescent.For PhB-PVA and PhB-PVAc films dye to dye interactions are also evidenced. In contrast with pHEMA films, fluorescence quantum yields are not constant at low concentrations. Instead, they decrease monotonically and are substantially lower than those obtained in pHEMA, showing that energy trapping takes place even in very dilute films. Singlet oxygen quantum yields could not be determined accurately in PhB-PVA films due to sensitivity limitations; however, 1O2 generation could be evidenced at the highest dye concentrations. Summarizing, dye to dye interactions and energy trapping govern photoactivity of dye-polymer films. However, highly efficient films are obtained using pHEMA, even in cases - such as those studied so far - in which the dye is distributed randomly into the polymer. As expected, efficiency is higher for RB than for PhB films.References1- Litman Y., Rodríguez H. B., San Román E., Photochem. Photobiol. Sci., 15, 80, 2016.